I. Technical Field
The present invention relates to a plasma display panel used for image display.
II. Description of the Related Art
Recently, plasma display panels (hereinafter referred to as “PDP”) have received attention as color display devices capable of having a large screen and being thin and light in weight.
An AC surface discharge type PDP typical as a PDP has many discharge cells between a front substrate and a rear substrate that face each other. The front substrate has the following elements:                a plurality of display electrode pairs disposed in parallel on a glass substrate; and        a dielectric layer and a protective layer that are formed so as to cover the display electrode pairs.In such a front substrate, each display electrode pair is formed of a pair of scan electrode and sustain electrode. The protective layer is a thin film made of alkali earth oxide such as magnesium oxide (MgO), protects the dielectric layer from ion spatter, and stabilizes the discharge characteristic such as breakdown voltage. The rear substrate has the following elements:        a plurality of data electrodes disposed in parallel on a glass substrate;        a dielectric layer formed so as to cover the data electrodes;        a mesh barrier rib disposed on the dielectric layer; and        a phosphor layer disposed on the surface of the dielectric layer and on the side surfaces of the barrier rib.        
The front substrate and rear substrate face each other so that the display electrode pairs and the data electrodes three-dimensionally intersect, and are sealed. Discharge gas is filled into a discharge space in the sealed product. Discharge cells are formed in intersecting parts of the display electrode pairs and the data electrodes. In a PDP having this structure, ultraviolet rays are emitted by gas discharge in each discharge cell. The ultraviolet rays excite respective phosphors of red, green, and blue to emit light, and thus provide color display.
A subfield method is generally used as a method of driving the PDP. In this method, one field period is divided into a plurality of subfields, and the subfields at which light is emitted are combined, thereby performing gradation display. Each subfield has an initializing period, an address period, and a sustain period. In the initializing period, initializing discharge occurs in each discharge cell, and a wall charge required for a subsequent address discharge is formed. In the address period, address discharge is selectively caused in a discharge cell where display is to be performed, thereby forming a wall charge required for a subsequent sustain discharge. In the sustain period, a sustain pulse is alternately applied to the scan electrodes and the sustain electrodes, sustain discharge is caused in the discharge cell having undergone the address discharge, and a phosphor layer of the corresponding discharge cell is light-emitted, thereby displaying an image.
The PDP is manufactured by a front substrate preparing process, a rear substrate preparing process, a sealing process, an exhausting process, and a discharge gas supplying process. In the sealing process, the front substrate prepared in the front substrate preparing process is stuck to the rear substrate prepared in the rear substrate preparing process. In the exhausting process, gas is exhausted from the space inside the PDP. Since the front substrate is stuck to the rear substrate using frit in the sealing process, they are superimposed on each other and are fired at the temperature of a softening point of the frit or higher, for example, at about 440° C. to 500° C.
Impure gas such as water (H2O), carbon dioxide gas (CO, CO2), and hydrocarbon (CnHm) is exhausted from the frit or the like, and part of the impure gas is adsorbed into the PDP. The air inside the PDP and the impure gas are exhausted in the subsequent exhausting process. However, it is difficult to completely exhaust all gases including the impure gas adsorbed in the PDP, and some impure gas inevitably remains inside the PDP. Additionally, as the screen size and definition of the PDP have been recently increased, the remaining amount of the impure gas is apt to increase.
However, it is known that the material of the protective layer or phosphor reacts with the impure gas and its characteristic degrades. Especially, significant water remaining inside the PDP adversely affects the discharge characteristic of the protective layer, reduces the breakdown voltage of the discharge cells, and causes a “bleeding” degradation of the image quality on the display screen, disadvantageously. When a still image is displayed for a long time, “burning into” is caused, i.e., the image becomes an afterimage, disadvantageously. The hydrocarbon reduces the surface of the phosphor, or degrades the light emission luminance of the phosphor, disadvantageously.
Therefore, it is one of the important issues that the impure gas remaining inside the PDP, especially water and hydrocarbon, is reduced, the discharge characteristic is stabilized, and variation with time is suppressed. As a method of removing the impure gas, an attempt where water is removed by disposing an adsorbent such as crystalline aluminosilicate, γ activated alumina, or amorphous activated silica inside the PDP is disclosed in Japanese Patent Unexamined Publication No. 2003-303555, for example. An attempt where water is removed by disposing a magnesium oxide film in a region other than the image display region inside the PDP is disclosed in Japanese Patent Unexamined Publication No. H05-342991. An attempt where hydrocarbon gas is removed by disposing an oxide or an adsorbent in a region other than the image display region inside the PDP is disclosed in International Publication No. 2005/088668 Brochure. Here, the adsorbent is produced by adding a platinum-group element as hydrocarbon decomposing catalyst to the oxide. The oxide is alumina (Al2O3), yttrium oxide (Y2O3), lanthanum oxide (La2O3), magnesium oxide (MgO), nickel oxide (NiO), manganese oxide (MnO), chrome oxide (CrO2), zirconium oxide (ZrO2), iron oxide (Fe2O3), barium titanate (BaTiO3), or titanium oxide (TiO2). Japanese Patent Unexamined Publication No. 2002-531918 discloses an attempt where a metal getter such as zircon (Zr), titanium (Ti), vanadium (V), aluminum (Al), or iron (Fe) is disposed on the barrier rib in the PDP and an organic solvent is absorbed.
In spite of these attempts, it is difficult to sufficiently remove impure gas such as water, hydrocarbon, or organic solvent, and it is difficult to suppress the degradation of the protective layer and phosphor.